Evaporation of a liquid and separation of liquid components has been investigated somewhat thoroughly. The generally accepted theory of evaporation is that the molecules of the substance are in continuous motion and are bound to each other by various molecular forces. The energy of the molecules follows a distribution, and statistical theory holds that the distribution is generally bell-shaped. Evaporation occurs when molecules having higher than average energy escape from an exposed surface of the substance when their velocity is too great to be restrained by the molecular forces. The molecules which escape from the exposed surface also have a distribution of energies and they exert a partial pressure corresponding to the physical and chemical nature of the evaporating substance and surrounding conditions such as pressure and temperature. The escaped molecules continue their motion, but being freed from the mother-liquid they are able to travel longer distances. The ability of these molecules to move in the gaseous stage is described in terms of molecular mean free path, which is the average distance between collisions. By traveling away from the liquid, the evaporated molecules build a mass transfer boundary layer where the concentration of the evaporated substance decreases rapidly with the distance from the mother-liquid.
The process of evaporation is used to carry out one or more of the following three major functions:
1. to increase the content of a certain substance in a surrounding or a passing flow (humidification); PA1 2. to reduce the thermal energy of a given substance (cooling); and PA1 3. to segregate components of a mixture ("liquid-liquid" separation).
Processes of humidification and evaporative cooling deal basically with water as the substance to be evaporated. Evaporative separation is widely used in the process of distillation. According to this process, substances having different boiling points may be separated by the process of evaporation since the substance having a lower boiling point, i.e., being more volatile, will evaporate at a much higher rate. A problem arises when at some point during distillation the components in the mixture arrive at a concentration which can not be changed by distillation, and distillation of this mixture results in production of a gaseous substance having components in the same proportions as that of the liquid mixture. Such a mixture is known as an azeotropic substance.
Membranes which permit selective diffusion of materials are also known. These membranes rely upon chemical transmission of a solvent through a semi-permeable membrane. When pressure is applied to liquid on the side of the membrane having a higher solute concentration, the flow of liquid may be stopped. The amount of pressure necessary to stop the flow of liquid is known as the osmotic pressure.
It is also known that gases and liquids will diffuse through a perforated material. These materials typically have a microporous structure, the diameter of the pores being on the order of one to several hundred microns.
A phenomenon of porous evaporation is known wherein a remarkable increase in mass transfer is obtained when a liquid is forced through a sintered metallic or ceramic plate having microcapillaries. The liquid is brought to the evaporative surface, and a spurt of evaporating molecules diminishes the laminer underlayer of the evaporate's carrier flow. The mass transfer coefficient in such a device is tens of times higher than that from a flat, open liquid surface. Unfortunately, the sintered plates used in such applications suffer from two major drawbacks. First, it is almost impossible to provide a device of reasonable size yet having a large mass transferring surface. Second, a high pressure is required to bring the liquid to the surface through a plate which is at least one-eighth inch thick.
It is an object of this invention to provide a method and apparatus to promote evaporation of a substance through a porous membrane.
It is a further object of this invention to provide a method and apparatus for causing a substance to be evaporated to enter the pores of a porous membrane so that the distance between the substance and an opposite side of the membrane is less than the thickness of the membrane.
It is a further object of this invention to provide a method and apparatus for easily separating the components of a mixture, especially an azeotropic one.